Speaking only to road tandems:
Drive chain: 1,500 - 2,000 mi
Drive chain wheels: 8,000 - 12,000 mi (assuming drive chain is changed out at .75%)
Cassette: 8,000 - 10,000 mi (assuming drive chain is changed out at .75%)
Synchronization chain: 3,500 - 4,000 mi.
Synchronization chain wheels: 20k+ w/rotation
Eccentric / captain bottom bracket: Varies by brand / material. Good square tapers 15,000+ mi.
Stoker bottom bracket: Varies by brand / material. Good square tapers 15,000+ mi.
Head set: With proper maintenance, Chis King will not require replacement.
Brake pads - rim brakes: 2-3 yrs when they get too hard to be efficient.
Brake pads - disk brakes: Varies based on use.
Brake shoes - drum brake: n/a
Rear tire: ~1,000 miles on moderate to hilly terrain.